Advancements in communication technologies has permitted the implementation, and popular usage, of multi-user radio communication systems. A cellular communication system is exemplary of such a radio communication system. Information signals generated during operation of a radio communication system are transmitted upon radio communication channels.
To convert the information signal into a form to permit its communication upon a communication channel defined in a radio communication system, a transmitting station modulates the information signal upon a carrier wave of a carrier frequency within the range of frequencies which defines, at least in part, the communication channel. Thereby, a baseband-level signal of which the information signal is formed is converted into a radio frequency signal corresponding to the frequency of the communication channel upon which the resultant, communication signal is to be transmitted to a receiving station.
A transmitter includes one or more up-mixing stages at which the baseband, information signal is up-converted in frequency to be of the selected radio frequency. The mixing stages include mixer circuits coupled to receive the information signal and an up-mixing signal with which the information signal is to be multiplied or otherwise combined to form an up-converted signal. When multiple mixing stages are utilized, an IF (intermediate frequency) signal is formed at a first, or first series, of the mixer stages. A radio frequency signal is formed at the final mixing stage.
The receiving station which receives a radio-frequency, communication signal transmitted thereto upon the radio communication channel must, analogously, convert the radio frequency signal to a baseband level. One or more down-conversion stages is utilized to down-convert the radio frequency signal to a baseband-level. Typically, an antenna transducer converts the radio-frequency communication signal transmitted upon the communication channel into electrical form. When initially converted into electrical form, the signal is of small amplitude. Amplifier elements forming a portion of the receiver of the receiving station amplify the received signal. Because of the amplification of the received signal, during down-conversion, such as at an IF stage of the receiver, the elements of the receiver must exhibit highly linear characteristics to ensure proper operation of the receiver.
Sensitivity and selectivity of a receiver define, in part, the performance of the receiver.
Sensitivity is the ability of a communications receiver to recover information in the presence of noise. The sensitivity of the receiver depends upon, inter alia, a noise figure, information bandwidth, and carrier-to-noise ratio requirements of a digital demodulator, forming a portion of the receiver. The noise figure is set by the front-end down converter of the receiver.
Selectivity is the ability of the receiver to isolate information amongst interfering signals. The selectivity of the receiver is determined by channel filtering and signal handling.
Conventional receivers include, for example, an active mixer for converting an IF receive signal to a baseband level. Such an active mixer forms, for example, a portion of an IQ demodulator. The active mixer of a conventional receiver is sometimes formed of a quad mixer, or Gilbert Cell mixer. However, such an existing mixer provides a linear output responsive to an input provided thereto over a relatively small range of input signals. If the input to the mixer is beyond the allowable range, the output of the mixer no longer is linearly related to the input. Performance of the receiver including such a mixer is hampered by the non-linearity of the mixer.
A mixer circuit which exhibits improved linearity characteristics would advantageously improve radio performance of a radio device.
It is in light of this background information related to mixer circuits that the significant improvements of the present invention have evolved.